The present invention relates to a water heater, and particularly to an improved heat transfer apparatus designed to increase the efficiency of heat transfer to the water flowing through the water heater.
As shown in FIG. 5, a typical conventional water heater includes a cold water inlet tube section coiled around a combustion chamber shell in which fuel is burned by a gas burner, a plurality of parallel tube sections interconnected by return bends and disposed within parallel fin plates provided at the top of the combustion chamber, and a hot water outlet tube section extending from the fin plates to the bottom side of the combustion chamber shell to discharge hot water. The fin plates absorb heat from the combustion gas flowing upwards parallel to the surface of the fin plates and transfer the heat to the parallel tube sections, primarily by radiation and secondarily by conduction. During combustion, air enters into the combustion chamber from the bottom thereof and the hot combustion gas escapes from the combustion chamber through the fin plates. It is found that a substantial amount of heat energy is lost during the operation of such a water heater. This is because the large space between the parallel tube sections (see FIG. 6) can not hinder the directly upward movement of the hot gas, so a lot of heat not yet being absorbed by the fin plates and the parallel tube sections, together with the quickly escaping gas, is wasted. Moreover, the rapid outgoing movement of the hot gas causes a rapid draft of air into the combustion chamber, which blows and adversely affects the flame produced at the burner. In order to mitigate such a problem, wind shields are provided at the top and bottom of the combustion chamber to partially cover the air inlet and the waste combustion gas outlet. However, these wind shields have introduced disadvantages such as poor air circulation causing incomplete combustion of the fuel, an inhibited the discharge of the waste gas, causing some waste gas and the water vapor present in the waste gas to be confined in the heater. When the heater becomes cold, the waste gas remaining in the heater then produces liquid droplets on the wall of the heater and the fin plates, thereby causing the heater parts to deteriorate within a short period of time.